This invention relates to a method of and apparatus for processing a shaped video signal that represents an object to add a simulated shadow.
A common video effect performed using a video production switcher involves displaying alphanumeric characters against a selected background scene. For example, the characters might represent the title of a news program and the background scene might be a still image of an event of current interest.
Generally, the production switcher receives a full screen digital foreground video signal representing the title characters in a field of a selected color, typically a saturated blue, and a digital key control signal that has a value of zero for all points of the video field that are outside the boundaries of the title characters, a value of one for points that are inside the boundary of a character, and a value between zero and one in transition regions at the edges of characters. The key control signal is similar to a video signal containing no hue or saturation information and having a high contrast, i.e. a video signal representing a white object in a black field. The key control signal defines the region of the field that is occupied by the title characters represented by the foreground video signal. If the foreground video signal and a background video signal are linearly mixed to form an output signal, with the contribution of the foreground signal to the output signal being directly proportional to the value of the key control signal, the key control signal determines opacity of the title characters in the composite image represented by the output signal. The foreground video signal is multiplied by the key control signal to provide a so-called shaped foreground video signal. Since a value of zero in current digital video standards represents black (after a black level offset has been subtracted),the shaped video signal represents the characters in a black field. The production switcher also receives a full field background video signal representing the selected background scene, and the background video signal and the shaped foreground video signal are applied to a mixer, which multiplies the background signal by the complement of the key control signal and additively combines the result with the shaped foreground video signal, resulting in a montage signal representing the characters against the background scene.
The result of the operation described in the preceding paragraph is generally a video signal that represents a flat image having no depth information, as if the characters had been simply painted on a screen bearing the background scene. However, an image that is generally more visually pleasing can be obtained by processing the signals so that the characters appear to cast a shadow on the background scene, whereby an impression of three-dimensional depth is imparted to the viewer. The existence of the shadow implies to the viewer that the characters are spaced from the background image and that they are illuminated, and the position of the shadow relative to the characters conveys information about the position of the imagined light source.
It is known to create this shadow effect by using a digital picture manipulator with both primary and secondary key channel memories. The key control signal, which defines the geometric region occupied by, e.g., title characters, is written into both key channel memories and the shaped foreground video signal representing the characters is written into a video channel memory. The contents of the video channel memory and the primary key channel memory are read out using the same address signal, and concurrently the contents of the secondary key channel memory are read out using an address signal that is offset, horizontally and vertically, from the address signal used to read the primary key channel memory. Accordingly, the output of the secondary key channel memory defines the same geometric shape as the output of the primary key channel memory but the shape defined by the output of the secondary key channel memory is offset slightly from the shape defined by the output of the primary key channel memory. The output of the secondary key channel memory is multiplied by a shadow density factor to provide a shadow signal, and the background video signal is multiplied by the complement of (one minus) the shadow signal to provide a modified background signal. In the modified background signal, the brightness of the background scene is reduced within the area occupied by the geometric shape defined by the output of the secondary key channel memory. The modified background signal is then combined with the output of the video channel memory, and the dimmed area of the background then appears to be a shadow cast on the background by the characters.
This method of adding shadow information is subject to disadvantage, in that it requires two key channel memories as well as a video channel memory. Further, use of a digital picture manipulator simply for addition of shadows does not make good use of the capabilities of such device, which can be used for a wide range of video effects, including those that require rotation about arbitrary axes. In a digital picture manipulator having a single key channel memory, shadows can be created by alternating key memory samples for use as both the key control signal and the shadow signal, but this technique is undesirable because it results in degradation of both the foreground image information and the shadow information.